The discovery will force theoreticians back to the drawing boards to try and explain how these ancient clusters are formed, according to the study's lead author Professor Duncan Forbes of Swinburne University.

"Although globular star clusters were first discovered in 1665, it has taken more than 340 years to fully appreciate all the different types of star clusters that are made in the universe," says Forbes.

The new category of star cluster is less massive than Ultra Compact Dwarf objects, which are large and very densely populated star clusters, possibly the remains of small galaxies torn apart by bigger ones. But it is more massive than Faint Fuzzies, which are large, less dense star clusters, thought to come from the merging of smaller clusters.

A third group known as globular clusters, are tight spheres of stars more than ten billion years old, possibly made from the collapse of star forming gas clouds.

"No single model for the formation of these star clusters can currently reproduce the diversity of structural properties we have observed for old star clusters."

"Maybe it's some other process we haven't thought of yet."

Challenging search

Forbes and colleagues used NASA's Hubble Space Telescope to examine three nearby galaxies, eventually finding several good candidates of the right sizes and masses to the fill the gap in the avoidance zone.

"Finding these things is quite challenging, which is why we haven't found them before," says Macquarie University's Dr Lee Spitler, who is a co-author on the paper.

The researchers used special algorithms that search images for anything that looks like a star cluster. They then singled out clusters they were interested in.

"Because they're so far away, they look like little points of light in the Hubble images," says Spitler.

"That makes it hard to tell them apart from a star in our own galaxy, or from a more distant background galaxy, which also looks like a point of light."

The authors undertook follow up observations with the Keck telescope in Hawaii, to determine the distance to each of these objects and confirm that these are star clusters around galaxies.

Ancient stars

Forbes and colleagues also measured the colour of the star clusters, finding the lower mass clusters tended to be red, while higher mass ones tended to be blue in colour.

"This suggests differences in the overall chemical composition of stars in the cluster," says Forbes, as well as providing clues to their age.

"We suspect all of these objects that we discovered are pretty old, more than ten billion years old," says Forbes.